Deposition of high-purity silicon via high-surface area gas-solid or gas-liquid interfaces and recovery via liquid phase

ABSTRACT

Solid silicon is deposited onto electrically heated deposition plates by the reduction reaction of gaseous trichlorosilane and hydrogen which are mixed and pumped across the surfaces of the plates. The plates can have a number of high-surface area geometries such as concentric cylinders, spirals, or repeating S-shapes. Once the desired amount of silicon has been deposited, the deposition plates are heated to above the melting point of silicon causing the deposited silicon to slide off the plates in the form of a crust due to gravitational force. The plates are left coated with a thin film of liquid silicon which contains any impurities leached from the plates. This film is melted off separately from the main silicon crust to avoid contamination of the latter and the plates are then ready for the next deposition cycle.

The present application claims benefit of U.S. provisional patent application No. 60/913,997 filed Apr. 25, 2007 which is hereby incorporated herein in its entirety.

BACKGROUND TO THE INVENTION

The majority of the world's supply of high purity electronics grade or solar grade silicon is produced using the so-called “trichlorosilane-Siemens” route, wherein a mixture of trichlorosilane and hydrogen gas are placed in contact with electrically heated feed rods of silicon in a pressurized reactor vessel know as a “Siemens reactor.” The diameter of these rods increases over time as silicon is deposited onto their surfaces from the gas mixture as a result of the reduction reaction caused by the high temperature of the rods. The extremely high purity requirements of the silicon make it necessary to deposit then new silicon onto feed rods of silicon, as deposition onto any other materials typically causes contamination of the silicon from those materials. Nevertheless this process is inefficient for a number of reasons including:

-   -   1. The rods have a relatively low surface area which is one of         the key determinants of the deposition reaction rate,     -   2. A large amount of electricity is required to keep an         increasingly large mass of silicon with a still relatively low         ratio of surface area heated to the correct temperature for an         extended period of time,     -   3. It is very labor-intensive to remove the rods. The entire top         section of the reactor, which is shaped like a bell, has to be         unbolted and lifted to access the rods. The rods must then be         removed, and transported to a separate location for cutting         and/or crushing and packaging, or melting into ingots. This         excessive amount of handling results in high down-times for the         reactors during each batch cycle and can also introduce         impurities into the silicon. And,     -   4. New feed rods must be manufactured and reinstalled into the         reactors for the cycle to recommence.

Information relevant to the present invention can be found in U.S. Pat. Nos. 2,893,850, 4,242,307, 4,265,859, 4,272,488, 4,590,024, 4,710,260, 4,981,102, 5,006,317, 6,395,249, 6,861,144, 4,176,166, 2,904,404, 2,943,918, 3,016,291, 3,071,444, 3,168,422, 3,733,387, 3,865,647, 4,054,641, 4,710,260, 2,962,363, 4,125,592, 4,127,630, 4,242,697, 4,246,249, 4,282,184, 4,314,525, 4,353,875, 4,547,258 and US Patent Publication No. 2005-0201908 and non-US Patents: WO03106338A1 (PCT), 1292640 (DE), 2002-176653 (JP laid open pub. no.) and 37-17454 (JP); each of the foregoing United States Patents and non-United States Patents are hereby incorporated herein by reference. Each one of these referenced items, however, suffers from one or more of the limitations cited above.

SUMMARY

The present invention overcomes both the limitations of low deposition surface area per reactor and long and laborious rod changeover procedures associated with existing so-called Siemens reactors while still meeting the necessary purity requirements for the recovered silicon. Low surface area is overcome by using deposition plates manufactured from materials that can easily be fabricated into high-surface area geometries such as silicon carbide, silicon nitride, tungsten, and composites thereof. These materials also maintain their structural integrity at temperatures above the melting point of silicon allowing the deposited silicon to be melted off the plates, thus significantly reducing the time required to remove the silicon from the reactors and prepare the reactors for the next deposition cycle.

Therefore the main financial advantages of the present invention can be summarized as:

-   -   1. Significant reduction in electricity usage per quantity of         silicon produced,     -   2. Significant reduction in labor per quantity of silicon         produced,     -   3. Significant reduction in plant equipment costs per quantity         of silicon produced; fewer hydrogen deposition reactors are         required to produce the same amount of silicon,     -   4. In another preferred embodiment, where the silicon crust         which has slid off the deposition plates is melted completely in         the bottom of the deposition reactor and is cast into a         multicrystalline ingot inside the deposition reactor or is         pumped to a Czokralski crystal puller, elimination of the need,         and costs thereof, to remove, process, package, ship, unpack,         load, and re-melt the silicon in another location,     -   5. In another preferred embodiment, where liquid silicon         droplets fall from the plates and are solidified into beads by         contact with gaseous and/or liquid trichlorosilane and/or         silicon tetrachloride, elimination of the need, and costs         thereof, of crushing the recovered silicon into evenly sized         chunks or granules, and     -   6. In another preferred embodiment, where gaseous hydrogen,         trichlorosilane, and/or silicon tetrachloride are bubbled up         through liquid silicon and the liquid silicon is then either         solidified into an ingot of polycrystalline silicon or pumped         through appropriate piping to a Czokralski crystal puller,         elimination of the need, and costs thereof, to remove, process,         package, ship, unpack, load, and re-melt the silicon in another         location.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the top views of several deposition plate geometries that can be used to increased surface area in given volume of space.

FIG. 2 is a closeup of the plate surface, showing the plate material itself, the deposited layer of silicon, and the space between the deposited layers of silicon where the gas mixtures flows.

FIG. 3 shows a deposition reactor during the silicon deposition step.

FIG. 4 shows a deposition reactor during the silicon recovery step.

FIG. 5 shows a deposition reactor during the silicon removal step.

FIG. 6 shows a deposition-drip reactor with gaseous silane feed.

FIG. 7 shows a deposition-drip reactor with liquid silane feed.

FIG. 8 shows a deposition-bubble reactor with silane-hydrogen feed through

DESCRIPTION

Deposition Plates

As defined in this patent application, the term “deposition plates” refers to the surfaces upon which the silicon is deposited; therefore, as an example, a flat electrical conducting plate may create at least two deposition plates as discussed below (i.e. deposition surfaces) having a gas flow area between them. However, nothing precludes the use of more than one plate to form deposition plates; for example, using two vertical flat electrical conducting plates by placing them next to one another to form a gas flow channel. The electrical conducing plates need to have the following characteristics:

-   -   1. Good electrical conductivity     -   2. Good structural strength against repeated and extended         exposure to temperatures above the melting point of silicon, and         ability to support the mass of silicon to be deposited     -   3. Relative ease of fabrication     -   4. Compatibility with silicon (i.e., plate surface material         should minimize contamination of the silicon)         As an example, preferred materials meeting these requirements         include tungsten, silicon carbide, silicon nitride, graphite,         and alloys and composites thereof.

In a preferred embodiment, the deposition plates can be made by forming flat electrical conducting plates with typical thicknesses of several millimeters and typical widths of 1 to 2 meters, of the appropriate materials previously cited, into the shapes shown in FIG. 1. Preferably, any deposition plate geometries can be used to increased surface area in given volume of space. Most preferably deposition plate geometries are chosen to achieve the maximum surface area in given volume of space. While any geometry may be chosen; preferred examples of these geometries are 10 concentric cylinders, spiral plates 13, and S-shaped 18. In the preferred embodiment of this invention, the deposition plates are at least two vertically oriented deposition plates (i.e. two vertically oriented deposition surfaces).

In the preferred embodiment of this invention, FIG. 2 shows a cross-section of the details of the deposit plate including the deposited silicon 110, the electrically heated plate 50, and the gas flow area 150.

Deposition Reactor

The deposition reactor is largely similar to a conventional so-called Siemens reactor with the following exceptions:

-   -   1. Deposition occurs on electrically heated deposition plates         rather than on electrically heated pure silicon rods. The plates         can be heated either by direct application of electric current         or by induction heating.     -   2. There is a pressure plate that separates the reactor into two         sections during the deposition step. During this step, the upper         section of the reactor is pressurized with an incoming gas         mixture of hydrogen and trichlorosilane while the lower section         containing a hydraulically mounted silicon recovery crucible is         idle and at atmospheric pressure.     -   3. During the silicon recovery step, the pressure plate is         opened and the hydraulically mounted recovery crucible is raised         to the deposition plates. Upon further heating of the plates to         above the melting temperature of silicon, the deposited silicon         slides off or melts off into the crucible.     -   4. During the silicon recovery step, the hydraulically mounted         recovery crucible is lowered to the bottom of the reactor where         it attaches itself. The bottom of the reactor is then unlocked         and they hydraulic piston lowers both the reactor bottom and the         crucible out of the reactor so that the silicon in the crucible         can be removed,

FIG. 3 shows one preferred embodiment of a deposition reactor as configured during the deposition step including the reactor vessel 300, a recovery crucible 320 in the retracted state, a pressure plate 350 in a closed position, gas inlets 360 for the deposition gas mixture, the deposition plates 50 connected to electric heating leads 390 for heating the deposition plates, and vent 380 wherein the deposition gas 20 flows and reacts across the deposition plates 50 to deposit high purity silicon and the reacted gas is removed through the vent 380 and the electric heating leads 390 heat the deposition plate 50 to achieve the temperature for silicon deposition. Preferably, the recovery crucible is movable and more preferably is movable using a hydraulic system. Similarly, the reactor vessel 300 may be opened to remove the contents, and preferably either the top 395 or the bottom 315 or both may be opened and closed. Preferably the gas mixture is hydrogen-silane mixture.

FIG. 4 shows a preferred embodiment of the deposition reactor during the high purity silicon recover step including the reactor vessel 300, a recovery crucible 320 in the transfer state, a pressure plate 350 in an open position, gas inlets 360 wherein the gas is turned off, the deposition plates 50 having deposited silicon thereon and connected to electric heating leads 390 for heating the deposition plates wherein the deposition plates 50 (not shown as coated with 370) are heated to achieve melt-off temperatures so that the high purity silicon 370 transfers to the recovery crucible. During the melting process, it is possible that impurities from the deposition plate may leach into the thin liquid layer of silicon between the plate and the silicon crust to be recovered. This layer essentially acts as a barrier between the plate and the silicon crust. Once the crust is recovered in the recovery crucible, the thin liquid layer of silicon can be removed from the plate by continued melting and disposal or the plate itself can be changed out and replaced with a clean plate. The original plate can be cleaned separately and returned to service for the next batch.

Deposition-Drip Reactor

The deposition-drip reactor is largely similar to a conventional so-called Siemens reactor with the following exceptions:

-   -   1. Deposition occurs on electrically heated deposition plates         rather than on electrically heated pure silicon rods,     -   2. The mixture of hydrogen, trichlorosilane and/or silicon         tetrachloride is introduced in the lower section of the reactor         allowing an upward-flowing stream of gas to form.     -   3. This gas stream cools the silicon droplets which are dripping         from the deposition plates and they become solid beads which         accumulate in the bottom of the reactor.     -   4. The accumulated solid beads of silicon are periodically         removed from the reactor by shutting off the gas flow and         opening a discharge chute.     -   5. Trichlorisilane and/or silicon tetrachloride can also be         introduced into the deposition-drip reactor in liquid form. This         liquid provides additional cooling for the liquid droplets of         silicon dripping into it, allowing them to solidify into beads         and settle to the bottom of the reactor. The liquid that is         vaporized as a result of contact with the liquid silicon         droplets is mixed with hydrogen being pumped into the reactor         above the liquid surface. This gas mixture then contacts the         deposition plates which are heated to above the melting point of         silicon, thus causing the formation of the liquid silicon         droplets in the first place.

The high purity silicon removal step from the deposition reactor is shown in FIG. 5 including the reactor vessel 300, a recovery crucible 320 in the retracted state, a pressure plate 350 in a closed position, gas inlets 360 for the deposition gas mixture are closed, the deposition plates 50 connected to electric heating leads 390 for heating the deposition plates are off wherein the bottom 315 is opened so that the recovery crucible 320 with the high purity silicon 370 may be removed.

In another preferred embodiment, a deposition drip reactor is shown in FIG. 6 including the reactor vessel 300, a chute 365, gas inlets 360 for the deposition gas mixture, the deposition plates 50 connected to electric heating leads 390 for heating the deposition plates, and vent 380 wherein the deposition gas 20 flows and reacts across the deposition plates 50 to deposit high purity silicon and the reacted gas is removed through the vent 380 and the electric heating leads 390 heat the deposition plate 50 to achieve the temperature for silicon deposition. Preferably, silicon beads 385 are formed by heating the deposition plate to a sufficient temperature so that high purity silicon liquefies and is dripped from a bottom edges of the plates 55 to the bottom of the reactor (created by the cooling effect caused by the contact of the downward-traveling droplets with upward-traveling silane-hydrogen gas mixtures). Preferably, the deposition gas mixture is periodically shut off and the silicon beads 55 are removed from the chute 365.

In another preferred embodiment, a deposition drip reactor is shown in FIG. 7 including the reactor vessel 300, a chute 365, gas inlets 362 for hydrogen, liquid inlets 345 for liquid silanes 325 the deposition plates 50 connected to electric heating leads 390 for heating the deposition plates, and vent 380 wherein the deposition gas 20 created by evaporated silane and hydrogen flows and reacts across the deposition plates 50 to deposit high purity silicon and the reacted gas is removed through the vent 380 and the electric heating leads 390 heat the deposition plate 50 to achieve the temperature for silicon deposition. Preferably, silicon beads 385 are formed by heating the deposition plate to a sufficient temperature so that high purity silicon liquefies and is dripped from a bottom edges of the plates 55 to the bottom of the reactor (created by the cooling effect caused by the contact of the downward-traveling droplets with upward-traveling silane-hydrogen gas mixtures and/or contact of the droplets with a pool of liquid trichlorosilane). Preferably, the deposition gas mixture is periodically shut off and the liquid silane evaporated so that the silicon beads 55 are removed from the chute 365. Examples of liquid silianes include, but are not limited to, trichlorosilane and silicon tetrachloride and other silianes that are known to those skilled in the art.

Deposition-Bubble Reactor

The deposition-bubble reactor is largely similar to a conventional electrically-heated silicon melting crucible with the following exceptions:

-   -   1. It is a pressurized and sealed vessel     -   2. There is an inlet assembly at the bottom of the reactor so         that a gas mixture of hydrogen, trichlorosilane, and/or silicon         tetrachloride can be pumped into a pool of liquid silicon         already residing in the reactor. The inlet assembly includes a         pattern of small nozzles so that the gas will form small, evenly         distributed bubbles in the liquid silicon. Alternatively, the         inlet assembly can be suspended from the top of the reactor and         lowered into the liquid silicon by means of a hydraulic piston.     -   3. There is an exit valve and piping at the top of the reactor         to carry away the gas that has bubble through the liquid         silicon.     -   4. There is a discharge valve and piping at the bottom of the         reactor to carry away the accumulated liquid silicon for         crystallization.

In yet another preferred embodiment, a deposition bubble reactor is shown in FIG. 8 including the reactor vessel 300, liquid gas inlets 367 for hydrogen-liquid silane mixture, electrical base heater 388, connected to electric heating leads 392 for heating the deposition plates, and vent 380 wherein the silane-hydrogen bubbles 340 react to form silicon and the reacted gas 399 is removed through the vent 380. Preferably, the deposition gas mixture is periodically shut off and the liquid silane evaporated so that the silicon beads 55 are removed from the chute 365. Wherein, the hydrogen-liquid silane forms a gaseous mixture of hydrogen and trichlorosilane and/or silicon tetrachloride bubbled up through a pool of liquid silicon in an appropriate container in the bottom of the reactor resulting in the reduction reaction occurring at the very high surface area formed by the interfaces between the liquid and all of the bubbles. Preferably, after the desired amount of silicon has been reduced from the silanes into the hydrogen-liquid silane flow is shut off and either cooling the pool of liquid silicon to form an ingot of polycrystalline silicon is allowed, or, in an alternative preferred embodiment, pumping the liquid silicon out of the reactor vessel is done through appropriately constructed piping, to a Czokralski crystal puller for production of monocrystalline silicon. Examples of liquid silianes include, but are not limited to Trichlorosilane, silicon tetrachloride and other silianes. 

1. A method for producing high-purity silicon comprising the steps of: a. creating at least two vertically oriented deposition plates compatible with silicon and the vertically oriented deposition plates have a geometry chosen to increase surface area to the volume of space occupied by the vertically oriented deposition plates toward the theoretical maximum wherein the vertically oriented deposition plates are electrically heated and each vertically oriented deposition plate has a surface, b. Placing said plates in a reactor vessel, c. flowing a pressurized a deposition gas mixture into the reactor vessel to deposit a reduced silicon onto the plate surface wherein the pressurized mixture flows through the spaces between the vertically oriented deposition plates and the vertically oriented deposition plates are heated to a surface temperature to optimize the reduction reaction of deposition gas mixture but below a temperature that would effect the structural properties of the solid silicon, d. Heating the plates quickly after the desired amount of reduced silicon is deposited so as to create a thin film of liquid silicon between the plate material and the rest of the solid silicon deposition layer to form a solid crust of deposited silicon, e. Removing the solid crust of deposited silicon from the vertically oriented deposition plates at the thin film of liquid silicon, through gravitational force. f. Continuing to heat the vertically oriented deposition plates after they have been separated from the solid crust of deposited silicon so that the thin layer of liquid silicon is removed and disposed wherein the plate surface becomes clean and may be used for another deposition cycle.
 2. The method for producing high-purity silicon in claim 1 wherein the silicon deposition gas mixture is selected from hydrogen and the group consisting of trichlorosilane, silicon tetrachloride, other silanes, and mixtures thereof.
 3. The method for producing high-purity silicon in claim 1 wherein the least two vertically oriented deposition plates are made from materials with the appropriate structural, conductivity, heat resistance, and silicon compatibility characteristics.
 4. The method for producing high-purity silicon in claim 3 wherein the least two vertically oriented deposition plates are made from materials selected from the group consisting of silicon carbide, silicon nitride, tungsten, certain graphite composites, and mixtures thereof.
 5. A method for producing high-purity silicon comprising the steps of: a. creating at least two vertically oriented deposition plates compatible with silicon and the vertically oriented deposition plates have a geometry chosen to increase surface area to the volume of space occupied by the vertically oriented deposition plates toward the theoretical maximum wherein the vertically oriented deposition plates are electrically heated and each vertically oriented deposition plate has a surface, b. Placing said plates in a reactor vessel, c. flowing a pressurized mixture of a deposition gas mixture into the reactor vessel to deposit a reduced silicon onto the plate surface wherein the pressurized mixture flows through the spaces between the vertically oriented deposition plates and the vertically oriented deposition plates are heated to a surface temperature to optimize the reduction reaction of the silicon-bearing gas but below a temperature that would effect the structural properties of the solid silicon, d. Heating the vertically oriented deposition plates quickly after the desired amount of reduced silicon is deposited so as to form a liquid silicon, e. Collecting the liquid silicon in an appropriate container within the reactor with the vertically oriented deposition plates partially submerged in the liquid silicon to provide the necessary heating f. extracting the liquid silicon in a controlled manner. g. Continuing to heat the vertically oriented deposition plates after they have been separated from the deposited silicon so that the thin layer of liquid silicon is removed and disposed wherein the plate surface becomes clean and may be used for another deposition cycle.
 6. The method for producing high-purity silicon in claim 5 wherein the silicon deposition gas mixture is selected from hydrogen and the group consisting of trichlorosilane, silicon tetrachloride, other silanes, and mixtures thereof.
 7. The method for producing high-purity silicon in claim 5 wherein the least two vertically oriented deposition plates are made from materials with the appropriate structural, conductivity, heat resistance, and silicon compatibility characteristics.
 8. The method for producing high-purity silicon in claim 7 wherein the least two vertically oriented deposition plates are made from materials selected from the group consisting of silicon carbide, silicon nitride, tungsten, certain graphite composites, and mixtures thereof.
 9. The method for producing high-purity silicon in claim 8 wherein extracting the liquid silicon in a controlled manner comprises removal of the vertically oriented deposition plates in a controlled manner including reducing the temperature of the vertically oriented deposition plates so as to result in crystallization of the liquid silicon into an ingot of polycrystalline silicon.
 10. The method for producing high-purity silicon in claim 8 wherein extracting the liquid silicon in a controlled manner comprises pumping the liquid silicon out of the container and reactor, through appropriately constructed piping, to a Czokralski crystal puller for production of monocrystalline silicon.
 11. A method for producing high-purity silicon comprising the steps of: a. creating at least two vertically oriented deposition plates compatible with silicon and the vertically oriented deposition plates have a geometry chosen to increase surface area to the volume of space occupied by the vertically oriented deposition plates toward the theoretical maximum wherein the vertically oriented deposition plates are electrically heated and each vertically oriented deposition plate has a surface a serrated lower edge, b. Placing said plates in a reactor vessel, c. flowing a pressurized hydrogen gas and liquid silane into the reactor vessel to deposit a reduced silicon on the coated surface wherein the pressurized mixture is sufficient to flow through the spaces between the vertically oriented deposition plates and the vertically oriented deposition plates are heated to a surface temperature to optimize the reduction reaction of trichlorosilane and above a melting point of reduced silicon, d. Removing the liquid reduced silicon from the vertically oriented deposition plates by allowing the liquid reduced silicon to drip from the serrated edges such that it forms droplets,
 12. The method for producing high-purity silicon in claim 11 wherein the liquid silianes is selected from the group of liquids consisting of trichlorosilane, silicon tetrachloride, other silanes, and mixtures thereof.
 13. The method for producing high-purity silicon in claim 11 wherein the least two vertically oriented deposition plates are made from materials with the appropriate structural, conductivity, heat resistance, and silicon compatibility characteristics.
 14. The method for producing high-purity silicon in claim 13 wherein the least two vertically oriented deposition plates are made from materials selected from the group consisting of silicon carbide, silicon nitride, tungsten, certain graphite composites, and mixtures thereof.
 15. The method for producing high-purity silicon in claim 14 wherein the serrated lower edge has a geometry to effect formation of evenly sized and spaced droplets.
 16. The method for producing high-purity silicon in claim 15 wherein the droplets have sufficient time to form individual silicon beads wherein such cooling is effected by contact of the downward-traveling droplets with silicon deposition-hydrogen gas mixtures and/or contact of the droplets with a pool of liquid trichlorosilane and/or silicon tetrachloride at the bottom of the reactor.
 17. A method for producing high-purity silicon comprising: a. Coupling an electrical base heater that is electrically heated to a reactor vessel, b. Coupling a liquid-gas inlet to allow a hydrogen-liquid silane mixture to flow into the reactor vessel; c. Heating the a hydrogen-liquid silane mixture to a temperature at which the vaporized silane and hydrogen react to form high-purity silicon in bubble that are formed in a liquid state; and d. Shutting off the hydrogen-silane mixture; and e. removing the high purity silicon.
 18. The method for producing high-purity silicon in claim 17 wherein removing the high purity silicon is accomplished by cooling the pool of liquid silane to form solid of polycrystalline silicon which may then be removed from the reactor vessel.
 19. The method for producing high-purity silicon in claim 17 wherein removing the high purity silicon is accomplished by pumping the liquid silicon out of the container and reactor, through appropriately constructed piping, to a Czokralski crystal puller for production of monocrystalline silicon.
 20. The method for producing high-purity silicon in claim 17 wherein the liquid silane is selected from the group of liquids consisting of trichlorosilane, silicon tetrachloride, other silanes, and mixtures thereof. 